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15.6: Polycyclic Aromatic Compounds

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    67320
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    Objectives

    After completing this section, you should be able to draw the resonance contributors for polycyclic aromatic compounds, such as naphthalene, anthracene, etc.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • polycyclic aromatic compounds
    Study Notes

    As their name indicates, polycyclic aromatic hydrocarbons are aromatic hydrocarbons which contain more than one benzenoid (i.e., benzene-like) ring. This section deals only with those compounds in which the benzenoid rings are fused together; in other words, compounds in which at least one carbon-carbon bond is common to two aromatic rings. Another type of polycyclic aromatic hydrocarbon contains two or more benzenoid rings joined by a carbon-carbon single bond. The simplest compound of this type is biphenyl, the compound from which PCBs (polychlorinated biphenyls) are derived.

    biphenyl and 2,4,2’,5’-tetrachlorobiphenyl

    Figure 15.3: Structures of biphenyl and a typical PCB

    Aromatic Compound with a single ring

    Aromatic Compounds with more than one ring

    Benzene rings may be joined together (fused) to give larger polycyclic aromatic compounds. A few examples are drawn below, together with the approved numbering scheme for substituted derivatives. The peripheral carbon atoms (numbered in all but the last three examples) are all bonded to hydrogen atoms. Unlike benzene, all the C-C bond lengths in these fused ring aromatics are not the same, and there is some localization of the pi-electrons.

    The six benzene rings in coronene are fused in a planar ring; whereas the six rings in hexahelicene are not joined in a larger ring, but assume a helical turn, due to the crowding together of the terminal ring atoms. This helical configuration renders the hexahelicene molecule chiral, and it has been resolved into stable enantiomers.

    Naphthalene, C10H8, m.p. 81 degrees C; anthracene, C14H10, m.p. 217 degrees C; phenanthrene, C14H10, m.p. 100 degrees C; chrysene, C18H12, m.p. 253 degrees C; pyrene, C16H10, m.p. 150 degrees C; corannulene, C20H10, m.p. 268 degrees C; coronene, C24H12, m.p. 442 degrees C; hexahelicene, C26H16, m.p. 230 degrees C

    Figure 2: Examples of Polycyclic Aromatic Hydrocarbons (PAHs).

    Exercises

    Exercise \(\PageIndex{1}\)

    1) The following molecule is an isomer of naphthalene. Is it aromatic? Draw a resonance structure for it.

    clipboard_e0562f56d597c20654c6424dcd3adbc45.png

    2) The following molecule is adenine. It has a purine core. Of the nitrogen in the core, how many electrons are donated into the pi system?

    clipboard_ecaa439ac3680a77c7c48792d78dfdb9e.png

    Answer

    1) It has 10 pi electrons which follows the 4n+2 rule making it aromatic.

    clipboard_e019b7e99d06f1e47ddcfd45c2d4fa83e.png

    2) There is only one nitrogen of the core that contributes a set of lone pair electrons as 2 pi electrons (in red). All of the other nitrogens are sp2 hybrized and contribute 1 pi electron each. In total, the core is aromatic with 10 electrons in the pi-system.

    clipboard_eadb9a40c439809e205d92488e93031b7.png

    Exercise \(\PageIndex{2}\)

    This is an isomer of naphthalene. Is it aromatic? Draw a resonance structure for it.

    Bond line drawing of naphthalene.

     

    Answer

    Yes, it is aromatic. 4n+2 pi-electrons.

    15_6_1_Answer.svg

     

    Exercise \(\PageIndex{3}\)

    The following molecule is adenine. It has a purine core. Of the nitrogen in the core, how many electrons are donated into the pi system?

    Bond line drawing of adenine.

     

    Answer

    There is only one nitrogen of the core that contributes to the pi-system (in red). With this one lone pair the core is aromatic with 10 electrons in the pi-system.

    15_6_2_Answer.svg

    Contributors and Attributions


    15.6: Polycyclic Aromatic Compounds is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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